Our ongoing in vivo and in vitro studies of functionally important cellular populations and of the role of individual proteins in effector functions of T-lymphocytes have obvious immunopharmacological implications. Our studies identify the cell types and key proteins that should be targeted for efficient immunomodulation when there is a clinical need to inhibit or to enhance the intensity of the effector phase of the immune response. Our ongoing studies are concerned with finding most efficient inhibitors of enzymes we have implicated in lymphocyte functions. We found that it is possible to influence cellular responses of intact cells using peptides which mimic inhibitory domains of known protein kinases and have demonstrated that such peptides are able to block T-cell responses. The problem of peptide proteolysis due to ecto-peptidase activities is expected to be dealt with by the use of hydrophobic tail-modified peptides to improve intracellular intake. A novel set of molecular targets for immunomodulation is being explored in studies of ecto-protein kinases, ecto-phosphatases, and ecto-ATPase. The data suggest that after adequate characterization of these ecto-enzymes and the development of specific peptide inhibitors, it will be possible to take advantage of the extracellular location of this set of ecto- phosphorylation/dephosphorylation enzymes.